1. Field of the Invention
The present invention relates to humidification and heater arrangements used to control the humidity of breathable gases used in all forms of respiratory apparatus ventilation systems including invasive and non-invasive ventilation, Continuous Positive Airway Pressure (CPAP), Bi-level therapy and treatment for sleep disordered breathing (SDB) conditions such as Obstructive Sleep Apnea (OSA), and for various other respiratory disorders and diseases.
2. Description of Related Art
Respiratory apparatus commonly have devices to alter the humidity of the breathable gas in order to reduce drying of the patient's airway and consequent patient discomfort and associated complications. The use of a humidifier placed between the positive airway pressure (PAP) device (or flow generator) and the patient mask, produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition in cooler climates, warm air applied generally to the face area in and about the mask, as may occur inadvertently by a leak, is more comfortable than cold air.
Many humidifiers types are available, although the most convenient form is one that is either integrated with or configured to be coupled to the relevant respiratory apparatus. While passive humidifiers can provide some relief, generally a heated humidifier is required to provide sufficient humidity and temperature to the air so that patient will be comfortable. Humidifiers typically comprise a water tub having a capacity of several hundred milliliters, a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the PAP device and a gas outlet adapted to be connected to a patient conduit that delivers the humidified pressurized gas to the patient's mask.
Typically, the heating element is incorporated in a heater plate which sits under, and is in thermal contact with the water tub.
The humidified air may cool on its path along the conduit from the humidifier to the patient, leading to the phenomenon of “rain-out”, or condensation, forming on the inside of the conduit. To counter this, it is known to additionally heat the gas being supplied to the patient by way of a heated wire circuit inserted into the patient conduit which supplies the humidified gas from the humidifier to the patient's mask. Such a system is illustrated in Mosby's Respiratory Care Equipment (7th edition, 2004, ISBN 0-323-022154) at page 97.
Such a heating method for the patient conduit may only provide poor heat transfer due to the wire locating itself along the conduit wall rather than in the main gas stream. A wire will also only give poor turbulent mixing due to its low profile. As a result heat transfer may be poor and the mixing of water vapour and gas may also be poor.
Alternatively the heating wire circuit may be located in the wall of the patient conduit Such a system is described in U.S. Pat. No. 6,918,389.
U.S. Pat. No. 6,918,389 describes a number of humidifier arrangements for supplying low relative humidity, high temperature humidified gas to the patient. Some of these arrangements include pre- or post-heating of the gas to reduce the relative humidity.
None of these prior art devices provides an entirely satisfactory solution to the provision of comfortable humidified gas to the patient, nor to the ease of construction, the hygiene requirements and to the energy and patient comfort requirements at startup.